If human civilization is to extend itself beyond our planet, it will need to take with it the plants, animals and microorganisms that can sustain a living ecosystem. Nick Nielsen argues in this compelling essay that preserving our own species into the remote future thus means preserving terrestrial biology as well, drawing sustenance from it and maintaining it long enough for Earthly systems — and ourselves — to evolve in the myriad environments that await us among the stars. Mr. Nielsen’s examination of future speciation continues his ongoing series on existential risk and the nature of human expansion. You can keep up with his thinking on his two blogs: Grand Strategy: The View from Oregon and Grand Strategy Annex, or follow him on Twitter, where he is @geopolicraticus.

by J. N. Nielsen

Some time ago on Twitter I wrote, “Astrobiology is island biogeography writ large.” As in the classic science fiction film This Island Earth, we know our world to be an island oasis of life in the midst of a dark and possibly barren cosmic ocean of space. Astrobiology, in seeking to understand the place of life in the universe, seeks to understand our oasis of life in a cosmological context. Perhaps we will be forced to reconcile ourselves with an unrelieved cosmic loneliness; perhaps we will find that life is plentiful in the universe; perhaps life will be found to be so plentiful that it seems likely that life on earth is a consequence of panspermia. Whatever the result of our search, whatever remarkable discoveries we make, complex multicellular life like ourselves is likely to require some kind of homeworld for its initial evolution, and these worlds are likely to be distributed across widely separated worlds. Astrobiology is the cosmic biogeography that can serve as the field guide to this archipelago of habitable worlds.

The existence of galactic habitable zones (GHZ) and circumstellar habitable zones (CHZ) [1] implies regions of greater and lesser habitability, and the distribution of stars, planets, moons and other matter within the GHZ and CHZ implies worlds of greater and lesser habitability. A recent paper on Superhabitable Worlds [2] has suggested that there may be planets or planetary systems more clement to life than the environment of Earth. This implies the possibility that, although Earth looks like a unique oasis in the darkness of space, it may represent a cosmic region of sub-optimal habitability. At very least, we have much to learn about habitability, given the present necessity of extrapolating from a single data point. It seems, however, than in spite of our ignorance of life elsewhere in the cosmos we must first attempt to map the habitable zones of the universe if we are to search for the life that would supervene upon these habitable zones. The resulting patterns of habitability and life that we will eventually be able to map will be our biogeography of the cosmos – a kind of biocosmography or bioastrography – and we will want to consider the relationship between forms of life that occur at nodal points of habitability, if there are any such relationships.

Biogeographers in discussions of species distribution distinguish between stepping-stones routes, single-step routes, and sweepstakes routes of species dispersal. A stepping-stones route is a gradual process that is integral with the evolution of a species, which expands its range as its population grows, slowly covering a landscape. A single-step route is when, “organisms cross a barrier in a direct, single event, not sequentially.” [3] A sweepstakes route is dispersal via a vector that is rare and unusual. Many islands are eventually colonized by sweepstakes routes, which accounts for their distinctive flora and fauna. A lizard that happens to ride a floating log to an island and finds another lizard of the same species with which to perpetuate the species has experienced a sweepstakes dispersal route. Mostly on islands, one finds insects and birds and marine mammals, and few larger species that cannot fly or swim to the island under their own power.

Astrobiology will need to make similar distinctions among cosmological stepping-stones routes, single-step routes, and sweepstakes routes. We have already begun to understand some of the potential dispersal vectors. We know that a certain amount of matter is exchanged between the planets of our solar system, and it is possible that microorganisms have hitched a ride between planets on rocks blasted off the surface of a planet by some enormous impact. Under conditions prevailing in our solar system, however, we cannot expect that complex multicellular life could expand from our homeworld in this way.

In a superhabitable world or solar system, as noted above, it might be possible for complex, multicellular organisms to follow a stepping-stones route to dispersal beyond their homeworld, and thereby attain a far higher degree of existential viability than they would enjoy if they had remained an autochthonous species of a single celestial body. Apart from superhabitable worlds, on sub-optimally habitable worlds the scenario of single-celled microorganisms living on a piece of ejected debris that eventually finds itself on a celestial body other than its homeworld would constitute a paradigm case of a sweepstakes route.

It is possible to imagine circumstances of superhabitable worlds or even superhabitable solar systems in which the means provided by industrial-technological civilization are not necessary to the dispersal of life to other worlds, and a single-step route may be facilitated by naturally occurring means. In our perhaps sub-optimally habitable solar system, however, this is not possible. For the complex, multicellular life that we know and love on Earth, the only method of extraterrestrial dispersion would be a single-step route, the only dispersal vector would be a spacecraft, and the only way to produce a spacecraft is through a relatively advanced industrial-technological civilization.

Thus the long term existential viability of the terrestrial biosphere is predicated upon the growth and expansion of industrial-technological civilization, which seems paradoxical. In the early stages of industrial-technological civilization, up to and including the present day, the expansion of industrial-technological civilization has come at a cost to the terrestrial biosphere. It has even been suggested that another mass extinction is taking place, an anthropogenic mass extinction, as a result of human activity on Earth. Nevertheless, a vital technological civilization is, at least in our solar system, a necessary prerequisite to the survival of any life derived from the terrestrial biosphere once the Earth passes its natural span of habitability.

It is not only human beings that benefit from space travel and settlement; existential risk mitigation affects every living thing on Earth. If human beings establish a permanent and self-sustaining presence off the surface of the Earth, such an outpost of terrestrial life could only achieve a self-sustaining ecosystem through the parallel presence of thousands if not millions of other terrestrial species (keeping in mind that the majority of these species will be microorganisms, millions of terrestrial species do not necessarily impose insuperable spatial requirements for an off-world settlement). When we go into space, we must take with us the plants and animals that we eat or otherwise rely upon for our existential viability.

If we fail to utilize the resources of our industrial-technological civilization to lift ourselves and our fellow terrestrial species off the Earth, all that has been achieved by the terrestrial biosphere will be lost (i.e., it is not only humanity and civilization that are lost should we succumb to existential risk), except for the possibility of some extremophile microorganisms that might ultimately survive the dissolution of the Earth’s biosphere if blasted into space.

The natural lifespan of the Earth will eventually lapse and come to an end, and after that the natural lifespan of the sun, too, will be exhausted and lapse, which is why Wernher von Braun said, “The importance of the space program is to build a bridge to the stars, so that when the Sun dies, humanity will not die. The Sun is a star that’s burning up, and when it finally burns up, there will be no Earth… no Mars… no Jupiter.” [4] While his expression of the idea is anthropocentric, we can see that any bridge to the stars must also be a bridge for other terrestrial species as well as ourselves. In short, interstellar travel is a dispersal vector for terrestrial biology.

Once our terrestrial biology is extended to other worlds – initially, other worlds in our solar system, and then other worlds orbiting other stars – it will be subject to unprecedented selection pressures, and in the long term these selection pressures will result in speciation specific to the new environments in which terrestrial species gain a foothold. In other words, terrestrial life will continue to evolve, and it will evolve on other worlds in a way that it does not and would not evolve on Earth. Speciation on a cosmic scale will be the result. We do not know and cannot predict the direction that life will take in its adaptive radiation throughout the cosmos.

It will not be until the first terrestrial seeds are planted in lunar soil in a greenhouse on the moon, or in Martian soil in a greenhouse on Mars, that we will know what terrestrial plants grow well in these soils and under these conditions. Only experience can teach us this, as the interaction of organism and environment, especially under novel conditions, is too complex to predict with certainty, and life is a paradigm of contingency, subject to the thousand natural shocks that flesh is heir to.

If this ignorance of the consequences of space settlement for our own biology sounds like I am making light of our knowledge and abilities – after all, human beings have been farmers for more than ten thousand years – the idea is precisely analogous to another challenge faced by space travel. We do not yet know, and cannot predict on the basis of present knowledge of science, technology, and engineering, what technologies will prove to be the most robust forms of propulsion for interstellar vessels. Among the many concepts for interstellar propulsion that have been proposed there remains not only all the science yet to be done to confirm or invalidate the concept, but also the building of specific technologies on the basis of this science, engineering particular vehicles employing these specific technologies, and then the testing and proving of these vehicles in the kind of conditions that will only be faced in flight. In the same way, we do not yet know what terrestrial plants and animals will prove to be robust partners in space exploration.

We may eventually treat our food supply and sustainable ecosystem as an engineering problem, but that will compound rather than limit the unknowns of speciation. Because of our anthropocentric moral standards, we will likely have less moral compunction about modifying other species for their use on space settlements or other worlds; even then, species modified for our use in artificial environments (i.e., on shipboard and space settlements) or on other worlds (initially, Mars and moons in our solar system, and then other planets around other stars) will be subject to a twofold selection process that is only likely to accelerate their adaptive radiation, viz. these two selection pressures being the artificial selection resulting from human genetic engineering of other species and the natural selection of novel environments not encountered by any terrestrial species that remains on Earth.

Notes

[1] Cf. e.g., Astrobiology of Earth: The emergence, evolution, and future of life on a planet in turmoil, Joseph Gale, Oxford: Oxford University Press, 2009, p. 33

[2] Superhabitable Worlds, Heller, René and Armstrong, John. Astrobiology. January 2014, 14 (1): 50-66. doi:10.1089/ast.2013.1088.

[3] Trans-oceanic dispersal and evolution of early composites (Asteraceae) Liliana Katinas, Jorge V. Crisci, Peter Hoch, Maria C. Tellería, María J. Apodaca, Perspectives in Plant Ecology, Evolution and Systematics, Volume 15, Issue 5, 20 October 2013, Pages 269-280

[4] This was quoted by Friedwardt Winterberg at the Icarus Interstellar Starship Congress, Day 2; I have been unable to locate a source for this quote.